Using regex how to perform string match to get a different date format

Question

I have two dateString formats like :date-f1:+2 and :date-f2:-1, if the incoming string is in date-f1:+2 format, I need to console.log as processedDate.format("YYYY-MM-DD"), but if the incoming string is in :date-f2:-1 format, I need to console.log as processedDate.format("DD/MM/YYYY"). I have tried the below, but getting an exception Cannot read properties of null (reading '1')", could someone please advise;

let dateString = ":date-f1:+2";  // or ":date-f2:-1"

    function datesParse(dateString) {
        
    let matchFormat = dateString.match(/^:date-f1:(|):date-f2:(?:([\-\+])([0-9]+)+)?$/);

        let processedDate = moment();
        if (matchFormat[1] === "-") {
            processedDate = processedDate.subtract(matchFormat[2], 'days');
            console.log("Date::"+processedDate.format("DD/MM/YYYY"));
        } else if (matchFormat[1] === "+") {
            processedDate = processedDate.add(matchFormat[2], 'days');
            console.log("Date::"+processedDate.format("DD/MM/YYYY"));
        } else if (matchFormat[1] === "-") {
            processedDate = processedDate.subtract(matchFormat[2], 'days');
            console.log("Date::"+processedDate.format("YYYY-MM-DD"));
        } else if (matchFormat[1] === "+") {
            processedDate = processedDate.add(matchFormat[2], 'days');
            console.log("Date::"+processedDate.format("YYYY-MM-DD"));
        }
        
    }
    
    datesParse(dateString);

If there are only two specific formats(:date-f1:+2 and :date-f2:-1) where literally nothing changes, then instead of regex use if else else for checking. Using regex here seems wasteful. — anotherGatsby
– anotherGatsby, Commented Apr 20, 2022 at 6:04
Agreed, but if we are using includes like below, then we need to check -f1 and + or - operator at the same time ..right ? let word = "-f1" let operator = "+" if (dateString.includes(word) && dateString.includes(operator)) { .... — soccerway
– soccerway, Commented Apr 20, 2022 at 6:14
So the operators after f1/f2 changes? like can there be 'f1:+number' and 'f1:-number' the same for f2)? and you want to capture the operator and the word? These weren't mentioned in the question, so please update the question. — anotherGatsby
– anotherGatsby, Commented Apr 20, 2022 at 6:38

anotherGatsby · Accepted Answer · 2022-04-20 06:59:59Z

1

I modified your regex and also added named capture groups, try this: /^:date-(?<word>f[12]):(?:(?<operator>[-+])(?<number>[0-9]+)+)?$/gm

Test here: https://regex101.com/r/cNNHA5/1

With capture groups you can access each of those captured elements easily:

let line = ':date-f1:+2';

let pattern = /^:date-(?<word>f[12]):(?:(?<operator>[-+])(?<number>[0-9]+)+)?$/gm

let match = pattern.exec(line)

console.log('word', match.groups.word)
console.log('operator', match.groups.operator)
console.log('number', match.groups.number)

and use them for checking like:

if (dateString.includes(match.groups.word) && dateString.includes(match.groups.operator))

so you dont have to create extra vars word and operator.

edited Apr 20, 2022 at 6:59

answered Apr 20, 2022 at 6:52

anotherGatsby

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2 Comments

soccerway Over a year ago

Thank you, I have upvoted and accepted, how can we build a regex patter

anotherGatsby Over a year ago

@soccerway to create a regex literal you can do const re = /ab+c/gm; to create a constructor use const re = new RegExp('ab+c');. Read more about regex here: developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/…

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